Lithography is the process of printing from specially prepared surfaces some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
Generally, two different types of lithographic printing plates prepared by electrophotography have evolved.
One type of printing plate is produced by the following steps: (i) uniformly electrostatically charging a photoconductive layer, such as a coating of zinc oxide photoconductive pigment dispersed in a resin binder, carried on a support by means of a corona-discharge, (ii) image-wise discharging said photoconductive layer by exposing it to electromagnetic radiation to which it is sensitive, (iii) applying electrostatically charged oleophilic resin-containing toner particles to develop the resulting electrostatic charge pattern either by positive or reversal development and (iv) fixing the toner to the photoconductive layer. Fixing is usually accomplished by the use of heat which causes the toner resin powder to coalesce and adhere to the photoconductive layer.
The photoconductive layer with the fused oleophilic image portions is then converted to a lithographic master by treatment with a conversion solution. The conversion step treats the photoconductive coating so that water receptive background areas are obtained. The ink receptive portions are the fused oleophilic toner images.
In another type of printing plate the toner image resulting from step (iii) is transferred from the photoconductive layer to a toner receiving plate on which the toner transfer image is then fixed. In this system the photoconductor can be reused after cleaning. The toner receiving plate does not need a photoconductive coating; any conventional lithographic coating will suffice. Depending on the coating used subsequent chemical treatment may be necessary to render the background areas water receptive.
An example of a toner receiving plate provided with a lithographic coating is described in EP A 405016. Generally high image quality. comprising i.a. high resolution, is set forth as a prerequisite for such lithographic printing plates. However, when such lithographic printing plates are produced according to the second electrophotographic process described above, one of the limiting factors in view of quality of the final printing plate obtained, is the transfer of the toner image from the photoconductive layer to the toner receiving plate.
As is generally known the overall accuracy or fidelity by which an original is reproduced in an electrophotographic process, is to a large extent determined by the characteristics of the toner developer used. This fact being known in itself, there have been several prior art proposals for the manufacture of fine toner particles and in particular for toner particles, having a size distribution which meets a well-defined classification.
In U.S. Pat. No. 3,942,979, U.S. Pat. No. 4,284,701, GB 2,180,948, EP A 0 255 716 and in particular in WO 91/00548 such classified fine developers have been described.
One of the problems encountered with such fine developers, is the reduced efficiency of transferring the fine particle toner-image layer from the photoconductive surface to the image receiving layer, such as a lithographic printing plate precursor.
Such problem is e.g. explicitely recognized in EP-A-354 531 wherein in the third paragraph it is stated that the conventional electrophotographic process works well with large toner particles, but that difficulties arise as the size of the toner particles is reduced. Image defects such as "halo defect", "hollow character" and "dot explosion" arise. Thus, high resolution images require very small particles, but high resolution image free of image defects have not been achievable using electrostatically assisted transfer.
In experiments it also has been noticed that when upon toner receiving plates as described hereinafter, toner images present on the photoconductive drum are transferred in a conventional electrophotographic transfer station, the transfer efficiency decreases substantially when fine toner particles are used, as is required for obtaining high resolution images.